Throughout this application, various publications are referenced by Arabic numerals within parentheses. Full citations for these publications may be found at the end of the specification immediately preceding the claims. The disclosures of these publications in their entireties are hereby incorporated by reference into this application in order to more fully describe the state of the art as known to those skilled therein as of the date of the invention described and claimed therein.
A family of compounds, the flavonoids, are selectively toxic to multidrug resistant cells. Several recent studies have linked the regulation of P-glycoprotein (Pgp) gene expression to the expression of the drug-metabolizing P450 genes, with the speculation that, in normal cells, P-glycoprotein may function in conjunction with the P450 enzymes in the detoxication of xenobiotics (34-36). Indeed, benzo[a]pyrene, an inducer and a substrate for cytochrome P450Ia1, is also a substrate for P-glycoprotein (37). While investigating the coordinated regulation of these genes following exposure of multidrug resistant (MDR) cells to inducers of P450 gene expression, it was observed that one of these inducers, .beta.-naphthoflavone (.beta.NF), was considerably more toxic to multidrug resistant cells than to their drug-sensitive counterparts. This collateral sensitivity to .beta.NF and other flavonoid compounds has now been investigated in several multidrug resistant cell lines.
The tendency for cells to develop resistance to chemotherapeutic agents remains a major obstacle to successful cancer treatment. Perhaps the most insidious form of drug resistance has been termed multidrug resistance (MDR), in which cells become cross-resistant to a variety of functionally unrelated chemotherapeutic agents. In laboratory models, this resistance is most often associated with a decrease in drug retention and an overexpression of P-glycoprotein (Pgp), a membrane protein which has been shown to mediate drug efflux (see 1 for review). A direct relationship between P-glycoprotein expression and chemotherapeutic resistance has also been observed in a variety of malignancies (2, 3), including leukemias and lymphomas (4-8), myeloma (9-11), breast cancer (12, 13), ovarian cancer (14) and neuroblastoma (15, 16).
Efforts to overcome multidrug resistance have focused on agents which "reverse" the phenotype by increasing intracellular drug concentrations in resistant cells (17-19). Although these reversal agents have been proposed to act by competing for P-glycoprotein-mediated drug efflux, this has not yet been proven, and the complexity of the multidrug resistant phenotype, as well as the multiple pharmacological activities of many of these agents (20) suggests that other mechanisms may also play a role in the resensitization of multidrug resistant cells (21). Known reversal agents include calcium channel blockers (e.g., verapamil) (22), calmodulin inhibitors (23), indole alkaloids (24), quinolines (25), steroid hormones (26, 27) and immunosuppressive agents (28, 29). Verapamil and cyclosporin A are currently in clinical trial; however, their usefulness is limited by toxic effects (21).
An additional characteristic of some but not all of the multidrug resistant reversal agents is that they can also be selectively toxic to drug-resistant cells in the absence of multidrug resistant drugs. This was first shown for verapamil (30), and has since been observed with other calcium channel blockers, calmodulin inhibitors (31), steroid hormones and nonionic detergents (see 32 for review). The mechanism responsible for this "collateral sensitivity" of multidrug resistant cells has not yet been defined, but, at least in the case of the calcium channel blockers, has been proposed to involve P-glycoprotein (33). However, the selective toxicity of these agents to multidrug resistant cells has not been exploited therapeutically, since the concentration of drug mediating collateral sensitivity is often even higher than that required for reversal (31).
Several flavonoids are up to 15-fold more toxic to multidrug resistant cell lines relative to their parental counterparts. The flavone moiety is critical for selective toxicity to cells possessing the multidrug resistant phenotype, since the closely related congener, 2,3-dihydroflavone, and other complex lipophilic drugs such as tamoxifen or diethylstilbestrol, are largely devoid of this activity. Cells selected for resistance to .beta.NF show a decrease in both P-glycoprotein levels and the multidrug resistant phenotype, as well as a markedly altered morphology. These results suggest that flavonoid compounds may be a useful adjuvant to antineoplastics to prevent the emergence of cells possessing the multidrug resistant phenotype.